In British Pat. No. 994,890, a process is described for the preparation of organic polyisocyanates in which urethane isocyanates represented by the general formula EQU R' (O--CO--NH--R--NCO).sub.n
in which
R' represents a monovalent or polyvalent organic group of a mono-functional or poly-functional hydroxyl compound, PA1 N represents an integer of from 1 to 6 and PA1 R represents a bi-functional organic group
Are reacted with excess diisocyanates either by heat alone or in the presence of metal carboxylates, metal chelates or tertiary amines as catalysts until the fall in the isocyanate content corresponds to complete reaction of the urethane groups present with the isocyanate groups. The exact constitution of the reaction products cannot be specified exactly according to the teaching of the said British Patent. From the isocyanate values measured in the reaction mixtures and the end products isolated from them, it is concluded that the reaction products consist substantially of allophanate polyisocyanates. On closer analytical study of the products obtained according to this process by IR-Spectroscopy and, particularly, by gel chromatographic investigation, it is found that a considerable proportion consists of isocyanurate polyisocyanates and uretdione polyisocyanates formed by the dimerization and trimerization of isocyanate groups which take place as side reactions. If the reaction is stopped when the isocyanate content reached the value calculated for complete allophanatization, one urethane group is left unreacted in the reaction mixture for every isocyanate group which has been used up in a side reaction.
The occurrence of trimerization and dimerization as side reactions in the reactions according to the teaching of British Pat. No. 994,890 is not surprising since, in the absence of catalysts, long reaction times at relatively high temperatures are required until the isocyanate content falls to the value calculated for a complete reaction of the urethane groups (e.g. 24 hours at 130.degree. to 135.degree. C. in Example 1, page 3, lines 49 to 51), and the formation of isocyanurates from allophanates and isocyanates or dimers of these isocyanates is well known in the literature as described by J. C. Kogon, Journ. Am. Chem. Soc., Vol. 78, 1956, pages 4911 to 4914.
Although the use of catalysts makes it possible for much lower reaction temperatures to be employed (compare page 2, lines 92-95 of the British reference), it has long been known that the catalysts described (metal carboxylates, metal chelates, tertiary amines) are excellent dimerization and trimerization catalysts for isocyanates, so that one might expect such side reactions to occur to a considerable extent in the reaction of urethane groups with isocyanates to form allophanates.
The side reactions to form dimers and trimers of polyisocyanates, which cannot be excluded in the process according to British Pat. No. 994,890, lead to reaction mixtures which differ from the corresponding pure allophanate polyisocyanates mainly by being less compatible with many polyhydroxyl compound, particularly with polyhydroxypolyacrylates of the kind used as reactants for polyisocyanates in the production of polyurethane resins.
The problem of preparing pure allophanate polyisocyanates, i.e. allophanate polyisocyanates which are not "contaminated" with dimeric and, particularly, with trimeric polyisocyanates, has already been referred to in German Auslegeschrifts Nos. 2,009,179 and 2,040,645. The processes described therein, however, are aimed at producing allophanate polyisocyanates which contain aromatically bound isocyanate groups. According to the teaching of these prior publications, such compounds can be obtained free from the above mentioned side products if the addition reaction leading to the allophanate polyisocyanate is carried out in the presence of alkylating substances as described in German Auslegeschrift No. 2,009,179 and optionally in the presence of certain metal compounds as catalysts as described in German Auslegeschrift No. 2,040,645. The processes according to German Auslegeschrift Nos. 2,009,179 and 2,040,645 are not however, suitable for the preparation of allophanate polyisocyanates having aliphatically and/or cycloaliphatically bound isocyanate groups. This is clear, for example, from the fact that the isocyanate content of a reaction mixture of a urethane and an aliphatic polyisocyanate falls to only an insignificant extent over a period of 50 hours at 160.degree. to 170.degree. C. When metal catalysts according to German Auslegeschrift No. 2,040,645 were used in such a reaction, only a 66% reaction was observed over a period of 35 hours at 110.degree. to 120.degree. C., and the reaction mixture was by that time severely discolored. In the known state of the art, therefore, there is no process available for the production of pure, light colored allophanate polyisocyanates having aliphatically or cycloaliphatically bound isocyanate groups.
It was an object of the present invention to provide such a process.
It was surprisingly found that this problem could be solved by carrying out the reaction between aliphatic or cycloaliphatic polyisocyanates and compounds containing urethane groups in the presence of certain acids which will be described in more detail below.